Lipases are indispensable for the bioconversion of lipids within an organism through the catalysis of a variety of reactions that include hydrolysis, alcoholysis, acidolysis, esterfication and aminolysis. In humans, several lipases have been identified which possess lipolytic activities that regulate levels of triglycerides and cholesterol in the body. Enzymes from this superfamily, include lipoprotein lipase (LPL), hepatic lipase (HL), and pancreatic lipase (PL). While all three enzymes hydrolyze lipid emulsions and have similar aqueous-lipid interfacial catalytic activities, they each possess unique properties and physiological functions. All three enzymes act preferentially on the sn-1 and sn-3 bonds of triglycerides, to release fatty acids from the glycerol backbone (Dolphin et al. (1992) Structure and Function of Apolipoproteins, Rosseneu, M. (ed) CRC Press, Inc, Boca Ratan, 295-362). However, while PL completes the hydrolysis of alimentary triglycerides, the LPL and HL enzymes hydrolyze triglycerides found in circulating lipoproteins.
Due to the insolubility of lipids in water, the plasma transports complex lipids among various tissues as components of lipoproteins. Each lipoprotein contains a neutral lipid core composed of triacylglycerol and/or a cholesterol ester. Surrounding the core is a layer of proteins, phospholipids, and cholesterol. The proteins associated with the lipoprotein comprise a class of proteins referred to as apoproteins (apo). Based on apoprotein composition and density, lipoproteins have been classified into five major types that include chylomicrons, high-density lipoproteins (HDL), intermediate-density lipoproteins (IDL), low-density lipoproteins (LDL), and very-low density lipoproteins (VLDL).
Lipoprotein lipase (LPL) is the major enzyme responsible for the hydrolysis of triglyceride molecules present in circulating lipoproteins. LPL is associated with the luminal side of capillaries and arteries through an interaction with heparin-sulfate chains of proteoglycans and/or by glycerol phosphatidylinostintol. With the help of the activator apo CII, LPL hydrolyzes triglycerides of lipoproteins to produce free fatty acids. Muscle and adipose tissue assimilate these fatty acids. Alternatively, the fatty acids can be bound to albumin and transported to other tissues. As the lipase hydrolyzes the triglycerides of the lipoprotein, the particles become smaller and are often referred to as lipoprotein remnants. Within the plasma compartment, LPL converts chylomicrons to remnants and begins the cascade requirements for conversion of VLDL to LDL.
In its active form, LPL is a glycosylated non-covalent homodimer, with each subunit containing a binding site for heparin and apolipoprotein (apo) CII, an activator protein required for LPL activity. In addition to hydrolysis of triglycerides, LPL can hydrolyze a variety of other substrates, for example, long and short chain glycerides, phospholipids and various synthetic substrates (Olivecrona et al. (1987) Lipoprotein Lipase Borensztajn, J. (ed) Evener Publisher, Inc., pages 15-58).
In addition to the lypolytic activity of LPL described above, LPL plays additional roles in lipid metabolism. After sufficient hydrolysis, lipoprotein lipase is released from proteoglycans and travels with the remnants of the chylomicrons or VLDL. In the plasma LPL may then act to sequester the remnant particles on surface proteoglycans. Subsequently LPL can act as a ligand for receptors such as the LDL receptor, LDL-receptor related protein, gp330, or the VLDL receptor. This interaction with the cell surface receptor facilitates the uptake and degradation of plasma lipoproteins by cells (Williams et al. (1992) J. Biol. Chem 267:13284-13292 and Nykjaer e tal. (1993) J. Biol. Chem. 268:15048-15055).
Furthermore, LPL expressed in macrophages has been implicated in the cellular uptake of lipoprotein lipids and fat soluble vitamins, the degradation of lipid-containing pathogens and cell debris, and the creation of fatty acids for the energy requirements of the cell.
Disruption of LPL activity has also been implicated in other biological functions including, for example, enhanced oxidative stress in blood cells, increased fluidity of the membrane components of these cells and increases the susceptibility of their mitochondrial DNA to structural alterations (Ven Murthy et al. (1996) Acta Biochimica Polonica 43:227-40).
Hepatic lipase (HL) has functions in lipid metabolism similar to those of LPL. HL is located on the surface of liver sinusoids through glycosaminoglycan links where it interacts with lipoproteins and hydrolyzes triglycerides into free fatty acids. Unlike LPL, the activity of HL does not require an activator, but its activity may be stimulated by apo E. Thus, the preferred substrates of HL are the triglycerides of apo E-containing lipoproteins, such as chylomicron remnants, IDL, and HDL. Furthermore, the actions of HL on HDL is important in the reverse cholesterol transport process, a mechanism thought to reduce excess accumulation of cholesterol in hepatic tissue.
Like LPL, hepatic lipase has also been implicated in the uptake and degradation of lipoprotein in the hepatic tissue. Evidence suggests that HL may interact with cell surface receptors, such as those described above, and direct hepatic cellular uptake of lipoproteins and lipoprotein remnants. (Chappell et al. (1998) Progress in Lipid Research 37:363-422).
In its active form, HL exists as a monomer comprising both triglyceride lipase activity and phospholipase activity. As with LPL, treatment with heparin, results in the release of HL from the cell surfaces. While glycosylation plays an important role in secretion and affinity of LPL, it does not seem to be crucial for HL activity.
Pancreatic lipase (PL) is synthesized in acinar cells of the exocrine pancreas along with its protein activator, colipase. The pancreatic duct transports glycosylated PL and colipase into the duodenum. PL does not become anchored to membrane surfaces like LPL or HL. Instead, the free monomer of PL interacts with colipase which helps to anchor the PL to the lipid-water interface where the enzyme completes the hydrolysis of alimentary triglycerides.
In summary, lipases play a key role in lipid metabolism by regulating levels of cholesterol and triglycerides and therefore influence major metabolic processes including effects on lipid and lipoprotein concentrations, energy homeostasis, body weight, and body composition-parameters. Each of these metabolic consequences has been associated with common diseases, such as, hypertriglyceridemia, atherosclerosis, obesity and various other disease states described further below.
Accordingly, lipases are a major target for drug action and development. Thus, it is valuable to the field of pharmaceutical development to identify and characterize previously unknown lipases. The present invention advances the state of the art by providing a previously unidentified human lipase enzyme.